Aqueous dispersions are used in a variety of applications in the automotive coatings industry. They advantageously provide reduced organic emissions, lower toxicity, and/or reduced fire hazards. The term “dispersion” may generally refer to a two phase system of a finely divided solid, liquid, or gas in a continuous medium. However, as used herein, “dispersion” particularly refers to two-phase systems of one or more finely divided solids, liquids or mixtures thereof, in a continuous liquid medium such as water or a mixture of water and various cosolvents. “Emulsion” as used herein refers to a dispersion of liquid droplets in a liquid medium, preferably water or a mixture of water and various cosolvents.
Aqueous dispersions may be used as electrodeposition coatings, primers, sealers, basecoats, and/or topcoats. Various binders may be used in aqueous dispersions, including but not limited to, epoxy based resins, acrylic resins, polyester resins, alkyds, polyurethanes, polyurethane adducts, and the like.
Electrodepositable primers are used in a coating process in which electrically charged coating particles are ‘plated’ or ‘deposited’ out of a aqueous dispersion in order to coat a conductive part. Electrodeposition or “e-coat” processes are advantageous both economically and environmentally, due to the high transfer efficiency and low levels of organic solvent.
Curable electrodepositable coatings have traditionally been relied upon to provide an initial coating layer responsible for imparting optimum corrosion resistance to complex metal parts such as automotive bodies. The improved corrosion resistance often associated with electrodeposition is often attributed to several factors. First, electrodeposition processes result in the production of an even, continuous coating layer over all portions of the metal part, even those areas that would be inaccessible to traditional spray processes. Second, curable electrodeposition coatings have traditionally used aromatic moiety containing epoxy-based resins as well as one or more lead containing pigments.
Until recently, it has not been necessary for electrodepositable primers to provide cured films having optimum performance properties with respect to appearance, weatherability and/or UV durability. However, the automotive industry's continuing desire for improvements in quality, efficiency and cost encourages the development of monocoat systems for metal parts, especially those made of steel. A commercially successful monocoat composition must (i) be capable of application via traditional electrodeposition processes, (ii) provide optimum corrosion resistance, (iii) provide optimum finished film properties with respect to appearance, weatherability, and UV durability, and (iv) low volatile organic content (VOC), especially with regard to the solvents traditionally used in the polymerization of individual thermosetting components.
In addition, the continuing drive for improvements in quality, cost, and efficiency mandates commensurate improvements in the manufacture of such optimum coating compositions.
Thus, the poor weatherability associated with aromatic containing epoxy based electrodeposition coatings has lead to an increased interest in the development of aqueous dispersions based on other polymers but which retain many of the advantages of traditional aqueous dispersions, especially those which are electrodepositable. Similarly, there is a desire to improve the manufacturing processes of aqueous dispersions, especially electrodepositable dispersions, without diminishing any of the advantages traditionally obtained.
For example, the manufacture of electrodepositable aqueous dispersions has traditionally required the removal of significant amounts of organic solvents, normally present as a result of the polymerization of the individual thermosetting components. Such polymerization solvents are normally not removed until after the formation of the desired dispersion or emulsion. As a result, traditional manufacturing processes require the removal or ‘stripping’ of significant volumes of polymerization solvent from the final aqueous dispersion.
Such stripping operations are disadvantageous for several reasons. First, they require the expenditure of costly energy to volatilize the organic solvents. Second, stripping operations render valuable manufacturing equipment unavailable for more profitable operations. Third, because they take many hours to make, products requiring the removal of organic solvent have significantly higher labor costs. Fourth, stripping operations result in large volumes of waste solvent that are not reusable due to contamination with water. The disposal of contaminated waste solvent is expensive and contributes to a higher cost for products requiring stripping. Finally, manufacturing processes requiring large stripping operations generally result in products having a lower % nonvolatile, leading to lower yields and higher shipping costs.
Unfortunately, higher raw material costs associated with certain polymers mandate that manufacturing costs be reduced as much as possible, especially with respect to polymers such as acrylics, epoxies, and polyurethanes. Accordingly, a commercially acceptable acrylic based aqueous dispersion, especially an electrodepositable aqueous dispersion, must be made by a manufacturing process which eliminates most, if not all, of the disadvantages associated with the removal of large volumes of organic solvent from finished aqueous dispersions.
In addition to higher manufacturing costs, prior art electrodepositable monocoat primers have lacked the durability required for commercially acceptable systems. It would be advantageous to retain the traditional benefits of electrodeposition application and manufacturing processes while obtaining significant improvements in durability, especially as measured by weatherometer (WOM) and QUV. Prior art systems have failed to provide an acceptable balance of corrosion and weatherability.
Finally, continuing environmental and cost concerns encourage the development of low VOC coatings. Accordingly, it is desirable to obtain an aqueous electrodepositable primer having a low VOC which can provide finished monocoat films having a glossy appearance, advantageous corrosion resistance, and good durability as measured by weatherometer and QUV. Such systems should also preferably be lead free, especially with respect to lead pigments.
It is thus an object of the invention to provide aqueous dispersions that provide improvements in VOC and durability as measured by WOM and QUV.
It is another object of the invention to provide electrodepositable aqueous dispersions that provide improvements in VOC and durability as measured by WOM and QUV.
It is a further object of the invention to provide low VOC aqueous electrodepositable primers made with the aqueous dispersions of the invention which provide cured electrodeposited films having acceptable corrosion resistance and improvements in appearance and durability as measured by WOM and QUV.